The present invention relates to a method for forming laminated structures and more particularly to a method for forming multilayer ceramic products using very thin greensheets and/or very thin greensheets with dense electrically conductive patterns.
Multilayer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, an MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have via holes, more typically called vias. Prior to lamination, the vias are typically filled with an electrically conductive paste, such as a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, conventional ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After via formation, metal paste screening, stacking and laminating, the greensheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the via forming, screening, stacking and laminating steps of this process.
In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers is well known. Greensheet thicknesses below 6 mils, in general, are very scarcely used due to a variety of reasons. Chief among these reasons is that the handling, punching, screening and stacking of greensheets thinner than about 6 mils pose tremendous technical challenges. In fact, the use of one to two mils thick ceramic greensheets, which are punched and screened, using traditional MLC technology does not exist in the prior art. The term thin greensheet as used herein means that the thickness of the greensheet can be anywhere from about 0.5 mil to about 6.0 mils. Production level screening and stacking of thin greensheets is not possible with the current technology as the thin greensheets tend to shrink a lot and distort during the process of making MLC packages.
In the MLC packaging industry, it is very common to use capacitor layers, power layers and ground layers. The capacitance necessary in a package depends on the design of the package and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is always striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half. Additionally the number of layers needed for capacitance in a package as well has been reduced by about 50 percent. The reduction in the number of layers is desirable, as it reduces the cost and the process of making the substrate.
Bischoff et al. U.S. Pat. No. 5,603,147, the disclosure of which is incorporated by reference herein, discloses the manufacture of capacitors wherein 1.5 mil thick layers are sequentially assembled upon a carrier. After formation of the ceramic laminate, the ceramic laminate is removed from the carrier.
Natarajan U.S. patent application Ser. No.08/950,222, filed Oct. 14, 1997, now U.S. Pat. No. 5,976,286, the disclosure of which is incorporated by reference herein, discloses a sublamination process wherein a punched thin ceramic greensheet is bonded to a punched and screened thick ceramic greensheet and then the sublaminate is screened with a metallic material. Subsequently, a further punched thin ceramic greensheet is bonded to the sublaminate followed by screening with a metallic material. This process continues until the desired number of greensheets have been laminated to form the sublaminate.
The method of the present invention enables the via forming, screening, stacking and handling of very thin green sheets and/or green sheets with very dense metallized patterns in the manufacture of multilayer ceramic packages. With the preferred embodiment, thin greensheets were stacked and bonded to thick greensheets and then the stack had vias formed to result in a substructure which yielded excellent stability in screening and enabled excellent handling and alignment in stacking. The greensheet may have electrically conductive features within them, such as, a via, or over them, such as, a line or cap, to name a few.